Thin film deposition techniques are widely used to build interconnects, plugs, gates, capacitors, transistors and other microfeatures in the manufacturing of microelectronic devices. Thin film deposition techniques are continually improved to meet the ever increasing demands of the industry because the microfeature sizes are constantly decreasing and the number of microfeature layers is constantly increasing. As a result, the density of microfeatures and the aspect ratios of depressions (e.g., the ratio of the depth to the size of the opening) are increasing. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms, and corners in deep depressions that have very small openings.
One widely used thin film deposition technique is chemical vapor deposition (CVD). In a CVD system, one or more reactive precursors are mixed in a gas or vapor state and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes a reaction between the precursors to form a solid, thin film at the workpiece surface. A common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction. CVD processes are routinely employed in many stages of manufacturing microelectronic components.
Atomic layer deposition (ALD) is another thin film deposition technique that is gaining prominence in manufacturing microfeatures on workpieces. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer of gas molecules A coats the surface of a workpiece W. The layer of A molecules is formed by exposing the workpiece W to a precursor gas containing A molecules and then purging the chamber with a purge gas to remove excess A molecules. This process can form a monolayer of A molecules on the surface of the workpiece W because the A molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The layer of A molecules is then exposed to another precursor gas containing B molecules. The A molecules react with the B molecules to form an extremely thin layer of solid material C on the workpiece W. Such thin layers are referred to herein as nanolayers because they are typically less than 1 nm thick and usually less than 2 Å thick. For example, each cycle may form a layer having a thickness of approximately 0.5-1.0 Å. The chamber is then purged again with a purge gas to remove excess B molecules.
Another type of CVD process is plasma CVD in which energy is added to the gases inside the reaction chamber to form a plasma. U.S. Pat. No. 6,347,602 discloses several types of plasma CVD reactors. FIG. 2 schematically illustrates a conventional plasma processing system including a processing vessel 2 and a microwave transmitting window 4. The plasma processing system further includes a microwave generator 6 having a rectangular wave guide 8 and a disk-shaped antenna 10. The microwaves radiated by the antenna 10 propagate through the window 4 and into the processing vessel 2 to produce a plasma by electron cyclotron resonance. The plasma causes a desired material to be coated onto a workpiece W.
Although plasma CVD processes are useful for several applications, such as gate hardening, they are difficult to use in depositing conductive materials onto the wafer. For example, when the precursors are introduced into the chamber to create a metal layer, a secondary deposit of the metal accumulates on the interior surface of the window 4. This secondary deposit of metal builds up on the window 4 as successive microfeature workpieces are processed. One problem is that the secondary deposit of metal has a low transmissivity to the microwave energy radiating from the antenna 10. After a period of time, the secondary deposit of metal can block the microwave energy from propagating through the window 4 and into the processing vessel 2. The secondary deposit of metal is also generally non-uniform across the interior surface of the window 4. Therefore, the secondary deposit of metal on the window 4 can prevent the plasma from forming or produce non-uniform films on the workpiece.
To reduce the effects of the secondary deposit of metal on the window 4, the interior of the reaction chamber must be cleaned periodically. For example, flowing ClF3 through the processing vessel 2 is one possible process to clean the window 4. This process, however, requires that the reaction chamber be cooled from a deposition temperature of approximately 400° C. to a cleaning temperature of approximately 300° C. The chamber is then purged of the cleaning agent and reheated back to the 400° C. deposition temperature. The cleaning process generally requires 3-4 hours to complete, and it may need to be performed frequently when depositing a metal onto the workpiece. Moreover, even after purging the cleaner from the chamber, residual molecules of the cleaner may remain in the chamber and contaminate the resulting film or otherwise disrupt the deposition process. Therefore, it has not been economical to use plasma vapor deposition processes for depositing certain types of metal layers or other conductive materials on microfeature workpieces.